Jean Neel

Industrial state-of-the-art of pervaporation and vapour permeation in the western countries

This work reviews the current trends of industrial pervaporation (PV) and vapour permeation (VP) in Europe and the USA after the great changes that have been occurring since 1995. More than simply being an up-to-date state-of-the-art, the paper provides key data and information about patented applications and current suppliers for PV and VP membranes and membrane separation systems. It also discusses several examples for PV and VP industrial applications and points out many inherent advantages of these separation processes as compared to other more conventional approaches, in particular in terms of very significant savings in energy and raw materials. On the basis of information provided by the leading industrial companies in the field, the past trends and future prospects of PV and VP are eventually analysed to help draw new guidelines for the promotion of these technologies in the near future.

Sorption, diffusion and vapor permeation of various penetrants through dense poly(dimethylsiloxane) membranes: a transport analysis

Abstract A detailed analysis of the transport of chloroform, isomeric butanols, methanol and water vapors at 40°C through dense silicone rubber membranes has been undertaken by the vapor permeation technique. The variation of fluxes versus upstream activity, coupled to sorption isotherms of the investigated compounds in silicone rubber at 40°C (determined by the swelling technique), offer the possibility to determine the sorption and diffusion steps in the overall permeant flux. It is shown that the chloroform isotherm follows the Flory-Huggins theory, while alcohols require a more sophisticated approach (Koningsveld and Kleinjtens type) in order to fit the isotherms. The water sorption isotherm shows a typical sigmoid shape which could not be fitted correctly by classical sorption equations. The diffusion coefficient of chloroform in PDMS appears to be almost unaffected by solvent concentration in the membrane, similarly to the results obtained with other good solvents in PDMS. Diffusion coefficients of alcohols and water show, however, a common tendency to decrease with their local concentration. An interpretation of the sorption isotherms based on the cluster integral of Zimm and Lundberg suggests some clustering behavior, which seems consistent with the diffusion coefficient variation. The clustering tendency of the solvent molecules in the PDMS matrix was related to their proton donating power, which follows the sequence: water > methanol > 1-butanol > 2-butanol > 2-methyl-2-propanol > chloroform.

Sorption of organic solvents into dense silicone membranes. Part 1.—Validity and limitations of Flory–Huggins and related theories

Sorption isotherms of various organic solvents and aqueous mixtures into dense polydimethylsiloxane (PDMS) membranes have been analysed and the results are compared with Flory–Huggins theory, already reported to hold for solvent sorption into elastomeric materials. The original Flory–Huggins equation, as well as more complex versions derived later by different authors (variable interaction parameter, elastic contribution, modified entropic contribution) have been screened; it appears that although excellent agreement is offered by the Flory–Huggins simplest expression for good PDMS solvents (e.g. hydrocarbons and halogenated hydrocarbons), none of the modified Flory–Huggins theories leads to satisfactory data interpretation for poor solvents (ketones, alcohols), unless at least three adjustable parameters are used for the enthalpic contribution term. Cluster formation at high solvent activity, considered on the basis of Zimm–Lundberg clustering analysis, is supposed to explain the observed deviations for poor solvents. A semi-empirical power law (Freundlich type) gives the best fit in the latter case.

The engaged species induced clustering (ENSIC) model: a unified mechanistic approach of sorption phenomena in polymers

Abstract The description of sorption equilibria of solvents in polymers is a key problem which has to be solved in order to achieve a correct understanding of transport processes through dense membranes. Up to now, this purpose is limited essentially to solvents in elastomers, which are described by Flory-Huggins thermodynamics theory, while sorption of gases in glassy polymers is usually based on a mechanistic one, the so-called dual mode model. Given the difficulties encountered as soon as polymer solvent peculiarities (non-regular mixing enthalpy, network elastic contribution) are taken into account, other types of mixtures can hardly be described by purely thermodynamic models unless complicated expressions are used. A simple mechanistic approach has been developed in order to circumvent these limitations; it is based on the assumption that insertion of a solvent molecule into the polymer solvent matrix will be governed by the intrinsic affinity of the solvent for either a polymer segment or an already sorbed solvent molecule. The model enabling cluster formation description has been named engaged species induced clustering (ENSIC). A series of data related to the most frequently used polymer dense membrane materials have been used in order to check the fitting efficiency of the newly developed expression; it is shown to give a very good description of sorption isotherms of many binary polymer solvent systems, including polar and apolar compounds, in either elastomeric, glassy or thermoplastic membrane materials. The physico-chemical interpretation of the two probabilistic insertion parameters used in the ENSIC model, as well as prospects concerning the extension to multicomponent systems or transport mechanisms simulations are discussed.

Pervaporation membranes endowed with catalytic properties, based on polymer blends

Abstract Membranes endowed with both catalytic and separative properties were designed by using the polymer blend concept. Two polymer blend systems were studied: the poly (acrylonitrile) (PAN)-poly (styrene sulfonic acid) (PSSA) and the poly (vinyl alcohol) (PVA)-PSSA blend. From miscibility and extraction studies, it appeared that PAN-PSSA could not be employed due to polymer segregation and extraction of PSSA in the liquid mixture. In the case of PVA-PSSA, miscibility was observed at high PVA content but, whatever relative polymer concentration in the blend, PSSA extraction occurred. To circumvent this extraction problem, PVA crosslinking was carried out by heat treatment. The catalytic and separative properties of the membranes were tested with an esterification reaction mixture. Good catalytic properties were observed. A rather good selectivity was also obtained, but not sufficient for the combined process. By casting the PVA-PSSA blend onto a hydrophilic GFT membrane and by subsequently crosslinking the blend, a composite membrane with both good catalytic and separative properties was obtained.

Preparation of membranes from polyacrylonitrile-polyvinylpyrrolidone blends and the study of their behaviour in the pervaporation of water-organic liquid mixtures

Abstract Membranes were made by blending polyacrylonitrile (PAN) and polyvinylpyrrolidone (PVP) in dimethylformamide, followed by an evaporation—coagulation phase to solidify the blends. Membranes of different structures were obtained by changing the preparation conditions: asymmetric membranes with finger-like macrovoids (low PVP content in the polymer and low solvent content in the precipitant bath), asymmetric membranes with honeycomb-like sub-structure, unskinned, granulous membranes (long precipitation in tetrahydrofuran), and dense film membranes (long evaporation time). The formation of these structures was interpreted on the basis of known mechanisms, except in the case of unskinned granulous membranes, where other hypotheses were proposed. Only dense membranes were sufficiently selective for the dehydration of water—organic liquid mixtures. Highest selectivity was obtained at a 1:1 composition of the blend. This type of membrane showed good performance with water—tetrahydrofuran mixtures. Its selectivity was lower but its permeability was higher with water—ethanol mixtures. Generally speaking, the membrane consisting of the 1:1 blend had a similar behaviour to that obtained by radiochemical grafting.

Application of Flory-Huggins theory to ternary polymer-solvents equilibria: A case study

Abstract The ternary equilibrium data of the following systems: benzene/heptane/NBR at 60 °C (system 1), heptane/isooctane/PE at 25 °C (system 2), ethanol/water/CA at 20 °C (system 3) and ethanol/water/ P(E-co-VAc) at 32 °C (system 4), already reported in the literature, have been used and experimental results compared to predictions offered by the Flory-Huggins theory applied to ternary mixtures (one polymer and two liquids), with constant interaction parameters and negligible elastic contribution. Polymer-solvent interaction parameters have been determined from swelling in pure liquid, while liquid-liquid interaction parameters have been estimated from liquid-vapour equilibrium data curve fitting. It is shown that the Flory-Huggins theory offers reasonable prediction in the case of apolar liquids in an elastomeric matrix (system 1), while approximate isotherm patterns with significant discrepancies are obtained with apolar liquids in a thermoplastic (system 2). The equilibrium data of polar liquids in either homopolymer or copolymer (systems 3 and 4) can hardly be achieved by Flory-Huggins theory, even when a variable liquid-liquid interaction parameter is used. Implications in ternary diagram simulations and possible prediction improvements are discussed.

Glass transition temperature regulation effect in a poly(vinyl alcohol)-water system

Abstract The temperature of crystallization and of melting of water in poly(vinyl alcohol) (PVA) membranes with various degrees of acetylation have been measured at different cooling and heating rates, and compared to the glass transition temperature (Tg) of the swollen materials. Above a certain critical concentration of water, c ∗ , when water begins to crystallize upon cooling, the Tg does not decrease according to the Fox equation, but remains constant. This phenomenon of regulation of the Tg and the origin of c ∗ are explained by the phenomenon of segregation of water in the amorphous phase during the process of crystallization. Finally, the effects of melting temperature depression and of broadening of the melting peak of ice in PVA—water systems, very similar to those observed in saccharose—water and alcohol—water mixtures, are explained by the phenomenon of dissolution, and not by the confinement effect.

Sorption of organic solvents into dense silicone membranes. Part 2.—Development of a new approach based on a clustering hypothesis for associated solvents

The difficulties encountered in fitting sorption isotherms of associated liquids (alcohols, ketones) in polydimethylsiloxane (PDMS) by Flory–Huggins and related theories have been reported in a previous work; cluster formation, expected to explain these deviations, has been examined according to various approaches already reported to take into account such a phenomenon (polycondensation in an inert matrix, dimer formation). Given the poor results obtained by these approaches, a new equation, based on elementary affinity constants between polymer and sorbed solvent species in a lattice graph has been tested. Results obtained by fitting the resulting two-parameter equation to alcohol–PDMS results show the ability of the proposed equation to fit precisely the isotherms. Furthermore, pictures of the polymer–solvent lattice can be easily obtained by this approach, by computer simulation. Generalizations of the novel approach to other polymer solvent systems as well as to multicomponent systems are discussed. Diffusion studies on the basis of this heterogeneous matrix picture could possibly be undertaken later.

Sorption and diffusion of solvent vapours in poly(vinylalcohol) membranes of different crystallinity degrees

Solvent sorption and diffusion are the key processes that control membrane performances in membrane processes. The sorption characteristic of water and ethanol vapours in poly(vinylalcohol) (PVA) membranes of different crystallinity degrees was measured by microgravimetry and the diffusion characteristic was calculated from the sorption kinetics at different water activities by curve fitting. The sorption isotherms for water vapour in membranes of 28, 37, 44 and 56% crystallinity degrees at 40°C obey the Flory equation based on the polymer lattice model. When the sorption extent was corrected by assuming that only the polymer amorphous phase is accessible to the penetrant, a unique Flory χ interaction parameter, 0.3, was obtained for all samples except for the 28% crystallinity sample. For the latter sample, the lower χ value (0.18) obtained can be explained by a change in the sorption behaviour of the original crystalline domains which may undergo partial destruction. The diffusion coefficient increases with the average water content in the membrane according to an exponential relationship characterized by a limit diffusion coefficient and a plasticization coefficient. The higher the crystallinity of the membrane, the lower the values of the limit diffusion coefficient and the plasticization coefficient. The ethanol sorption was also well described by the Flory-Huggins equation. The limit diffusion coefficient for water was two orders of magnitude larger than that for ethanol.